Today one of the basic problems of nondestructive testing of materials is the provision of methods and devices for detecting non-uniformities of magnetic materials, which would ensure a high rate, sensitivity and reliability of inspection.
The existing methods and devices for detecting non-uniformities of magnetic materials hinder the progress of technology and the rate of production, because they call for the use of costly equipment installed outside the production line. The need for advanced methods and devices is also due to increasingly stringent requirements as to the sensitivity to non-uniformities of materials when the testing is carried out at elevated temperatures of more than 100.degree. C.
The existing methods and devices for nondestructive detection of non-uniformities of magnetic materials may be divided into two categories:
(1) methods and devices for ultrasonic testing; PA1 (2) methods and devices for magnetic testing.
The methods which fall under the first category (cf. "Pribory dlya nerazrushayushchego controlya materialov i izdeliy" ("Instrumentation of Nondestructive Testing of Materials and Articles"), ed. by V. V. Klyuyev, Machinostroyeniye Publishers, Moscow, 1976, pp. 201-234) are based on the excitation of ultrasonic vibration in the material under investigation. The ulrasonic vibration interacts with non-uniformities in the material, and the resultant signals are recorded.
This method is carried out with the aid of devices of the type that comprises a material to be investigated, piezoelectric converters and means to provide an acoustic contact with the surface of the material.
The ultrasonic vibration is produced and received by the piezoelectric converters which are of the contact or immersion type.
The foregoing method can be also be carried out with the aid of a device where electromagnetic-acoustic converters are used instead of the piezoelectric converters. Such a device includes a magnetizing system and a conductor through which current is passed and which extends in a magnetic field in parallel with the surface of the material under inspection. Ultrasound is produced by the action on the surface of the material being tested of electromagnetic oscillation coming from the conductor.
The methods and devices of the first category necessitate an acoustic contact between the piezoelectric converters and the article being tested; it is also necessary that the piezoelectric elements should be pasted to prisms. These factors put limitation on the temperature of the article under investigation and on the rate of testing. The application of electromagnetic-acoustic converters is limited by their low sensitivity which depends, among other things, on specific characteristics of artcles being tested, such as the dynamic magnetostriction constants.
The magnetic testing methods which fall under the second category (cf. "Pribory dlya nerazrushayushchego controlya materialov i izdeliy" ("Instrumentation for Nondestructive Testing of Materials and Articles"), ed. by V. V. Klyuyev, Machinostroyeniye Publishers, Moscow, 1976, pp. 30-71) are based on detecting and recording local distortions of the magnetic field produced by the magnetizing system in the presence of non-uniformities in the material being tested. Such methods are effected with the aid of devices of the type that comprises a material to be tested, a magnetizing system (inductance coils, ferroprobes, Hall elements, etc.), magnetosensitive elements and an indication system.
The methods and devices of the second category show a low sensitivity in detecting internal non-uniformities. This is due to a low signal-to-noise ratio which, in turn, is due to the impossibility of separating the useful signal from the magnetostructural noise. In addition, such methods involve the difficulty of converting constant magnetic fields of non-uniformities to high-frequency signals with the use of inductance coils, ferroprobes, etc.
There is known a defect detection method and device (cf. W. Lord and D. J. Oswald, Leakage Field Methods of Defect Detection, in International Journal of Nondestructive Testing. 1972, Vol. 4, pp. 249-274).
According to this method, a material to be tested is placed in a steady magnetic field which is partially distorted by a local non-uniformity in the material to extend as far as above the surface of the article being tested, i.e., to produce a leakage field.
The above method is carried out with the aid of a device comprising a magnetic material, a magnetizing system which is a permanent magnet or an electromagnet, as well as a magnetosensitive element which converts the local non-uniformity field to an electric signal, an amplifier and a recording system.
A non-uniformity or defect in the material being tested causes a redistribution of the magnetic field, which is detected by the magnetosensitive element.
The above method and device are disadvantageous in their low sensitivity in the case of internal non-uniformities, as well as low reliability of testing due to a low signal-to-noise ratio.
There is further known a method for detecting non-uniformities of magnetic materials (cf. French Pat. No. 2,053,185, Cl. G 01/n and G 0.1/n 29/00, filed on July 24, 1970), comprising placing a material to be tested in a magnetic field and acting by vibration on the surface of a magnetostrictive medium arranged in proximity to the non-uniformity zone of the material, which is followed by recording signals resulting from the electromagnetic-acoustic conversion taking place in the medium. Ultrasonic vibration is used to act on the surface of the material under investigation which is the magnetostriction medium.
Ultrasonic vibrations propagate in the magnetic material to interact with a non-uniformity. This results in an electromagnetic-acoustic conversion due to the magnetostriction, which means that the steady magnetic fields of the non-uniformities are modulated by the ultrasonic vibrations at an ultrasonic frequency to become variable magnetic fields which are recorded.
The latter method is carried out with the aid of a device which comprises a magnetizing system whereof the magnetic flux is passed through a magnetostrictive medium found in proximity to the non-uniformity zone of the material under investigation. The device further contains two converters mounted on the surface of the magnetostrictive medium and electrically coupled to a main radio-wave pulse generator and to a main amplifier connected to an indicator. One of the converters is piezoelectric and connected to the main radio-wave pulse generator. The second converter is an inductance coil which is connected to the main amplifier.
The main radio-wave pulse generator excites the piezoelectric converter which generates ultrasonic vibrations in the material being tested. The ultrasonic vibrations change the steady magnetic fields into variable fields which are detected by the inductance coil and then amplified and recorded.
The latter method and device are disadvantageous in that they are incapable of detecting internal non-uniformities. This is due to the fact that the variable magnetic fields caused by internal non-uniformities are screened because of the skin effect. Besides, this method does not make it possible to test materials with relatively high dynamic magnetostriction levels, which accounts for a limited range of materials that can be tested. If the device for effecting this method makes use of piezoelectric converters, provision has to be made for an acoustic contact of these converters with the material being tested. However, such a requirement imposes limitations on the rate of testing and on the temperature of the material.